The present invention relates generally to global navigation satellite systems, and more particularly to method and apparatus for mitigation of scintillations in signals of global navigation satellite systems caused by ionospheric irregularities.
Global navigation satellite systems (GNSSs) can determine target parameters, such as position, velocity, and time (PVT). Examples of currently deployed global navigation satellite systems include the United States Global Positioning System (GPS) and the Russian GLONASS. Other global navigation satellite systems, such as the Chinese Beidou and the European GALILEO systems, are under development. In a GNSS, a navigation receiver receives and processes radio signals transmitted by satellites located within a line-of-sight of the receiver. The satellite signals comprise carrier signals modulated by pseudo-random binary codes. The receiver measures the time delays of the received signals relative to a local reference clock or oscillator. Code phase measurements enable the receiver to determine the pseudo-ranges between the receiver and the satellites. The pseudo-ranges differ from the actual ranges (distances) between the receiver and the satellites due to an offset between the time scales of the GNSS and the receiver. If signals are received from a sufficiently large number of satellites, then the measured pseudo-ranges can be processed to determine the coordinates and the offset between the time scales of the GNSS and the receiver. This operational mode is referred to as a stand-alone mode, since the measurements are determined by a single receiver. A stand-alone system typically provides meter-level accuracy of positioning.
To improve the accuracy of positioning, differential navigation (DN) systems have been developed. In a DN system, the position of a user is determined relative to a base station, also referred to as a base. The base is typically fixed, and the coordinates of the base are precisely known; for example, by surveying. The base contains a navigation receiver that receives satellite signals and that can determine the corrections to GNSS measurements based on the known base position. In some DN systems, the raw measurements of the base can serve as corrections.
The user, whose position is to be determined, can be stationary or mobile; in a DN system, the user is often referred to as a rover. The rover also contains a navigation receiver that receives GNSS satellite signals. Corrections generated at the base are transmitted to the rover via a communications link. To accommodate a mobile rover, the communications link is often a wireless link. The rover processes the corrections received from the base, along with measurements taken with its own receiver, to improve the accuracy of determining its position. Accuracy is improved in the differential navigation mode because errors incurred by the receiver at the rover and by the receiver at the base are highly correlated. Since the coordinates of the base are accurately known, measurements from the base can be used for calculating corrections, thus compensating the errors at the rover. A DN system provides corrections to pseudo-ranges measured with code phase.
The position determination accuracy of a differential navigation system can be further improved if the pseudo-ranges measured with code phase are supplemented with the pseudo-ranges measured with carrier phase. If the carrier phases of the signals transmitted by the same satellite are measured by both the navigation receiver in the base and the navigation receiver in the rover, processing the two sets of carrier phase measurements can yield a position determination accuracy to within several percent of the carrier's wavelength. A differential navigation system that computes positions based on real-time carrier phase pseudo-range measurements, in addition to the code phase pseudo-range measurements, is often referred to as a real-time kinematic (RTK) system. Processing carrier phase measurements to determine coordinates includes the step of ambiguity resolution; that is, determining the integer number of cycles in the carrier signal received by the navigation receiver from an individual satellite.
The accuracy with which target parameters can be determined using GNSS signals is affected by various factors; in particular, by the propagation of the satellite signals through the ionosphere. The ionosphere is a dispersive media located approximately between 40 and 1000 km above the Earth's surface. It is saturated by electrically charged particles (electrons and ions). The highest concentration of charged particles is within 250-400 km above the Earth's surface. Since the ionosphere is a dispersive media, it influences both the group delay and phase advance of radio signals, resulting in different values for the group velocity and the phase velocity of radio signals. The product of the group velocity and phase velocity in the ionosphere is equal to the speed of light squared.
The values of the group velocity and the phase velocity are dependent on the integral of the concentration of charged particles along the signal propagation path. The concentration of charged particles is characterized by the value of Total Electron Content (TEC). TEC is counted as the number of electrons in a tube of 1 m2 cross section extending from the transmitter to the receiver. In the case of GNSS, the transmitter is a GNSS satellite, and the receiver is GNSS navigation receiver.
As the TEC increases, the group velocity decreases, and the phase velocity increases. The TEC value is dependent on the state of the ionosphere and the obliquity factor. The state of the ionosphere strongly depends on the sun. When the sun rises, its radiation breaks up gas molecules into ions and electrons. The electron density reaches its maximum around 2 pm local time. Then ions and electrons start to recombine, and, at night, the electron density declines to its daily minimum.
Earth seasonal variations also lead to variations in the TEC. The sun is higher above the horizon in summer than in winter; consequently, on average, the TEC is higher in summer than in winter. The TEC is also dependent on the geographical position on the Earth, as the elevation of the sun is different for different latitudes, and the Earth's magnetic field is different for different locations.
Similarly, the sun has its own seasonal variations, such that approximately every 11-12 years its activity reaches the maximum. Approximate years of maximum of solar activity are 2001, 2013, etc. The increased solar activity is characterized by frequent solar flares that eject plasma, including high-energy protons, accompanied by X-rays. Such solar flares are the reasons for high and non-uniform concentration of free electrons in the Earth's ionosphere. The most difficult ionospheric irregularities to predict are ionospheric irregularities of relatively small size, such that receivers separated by ˜1 km or less on the ground could receive satellite signals with noticeably different group delay due to the ionosphere. These relatively small size irregularities not only delay but also scatter radio signals, leading to rapid fluctuations in amplitude and phase; these rapid fluctuations are referred to as scintillations. In the most severe cases, scintillations can lead to complete loss of signal.
Method and apparatus for the mitigation of scintillations in global navigation satellite systems caused by ionospheric irregularities are therefore advantageous.